Fusion of calcium isotopes and of other medium-mass systems

Alberto Stefanini

INFN - Laboratori Nazionali di Legnaro, Italy

Heavy-ion fusion excitation functions near and below the Coulomb barrier are strongly influenced by couplings to static deformations, nuclear surface vibrations and nucleon transfer channels between the colliding nuclei.  Strong enhancements of the fusion cross sections, due to such couplings, were systematically observed in several experiments.  However, measurements of very small cross sections, down to 10-20 nb, for a number of systems, well below the low-energy limit of the fusion barrier distribution produced by channel couplings, have shown that the slope of the excitation function keeps increasing at lower energies, so that the cross sections fall strongly below the predictions of standard coupled-channels calculations; this was named a “hindrance” effect. Recent experiment on medium-light systems (A=30-50) indicate that hindrance shows up in all cases but with varying features, depending on the structure of the two nuclei and on their neutron excess, but not on the sign of the fusion Q value. For such systems, further interest lies in the vicinity to lighter cases where the fusion cross section at deep sub-barrier energies has far-reaching consequences in astrophysics, for the late evolution of massive stars.  In this context, the talk will focus on the recent measurements performed at INFN-Legnaro, concerning ⁴⁰Ca+⁴⁰Ca, ⁴⁰Ca+⁴⁸Ca and ⁴⁸Ca+⁴⁸Ca, whose behaviors below the barrier offer a nice opportunity to discuss all these features fully. Results on near-by systems will allow to build up a detailed framework of sub-barrier heavy-ion fusion dynamics in that mass region.